Thick film materials are mixtures of metal, metallic oxides, glass and/or ceramic powders dispersed in an organic medium. These materials, which are applied to nonconductive substrates to form conductive, resistive or insulating films, are used in a wide variety of electronic and light electrical components.
The properties of such thick film compositions depend on the specific constituents of the compositions. Most of such thick film compositions contain three major components. A conductive phase determines the electrical properties and influences the mechanical properties of the final film. A binder, usually a glass and/or crystalline oxide, holds the thick film together and bonds it to a substrate, and an organic medium (vehicle) acts as a dispersing agent and influences the application characteristics of the composition and particularly its rheology.
High stability and low process sensitivity are critical requirements for thick film resistors in microcircuit applications. In particular, it is necessary that resistivity (R.sub.av) of a resistor be stable over a wide range of temperature conditions. Thus, the thermal coefficient of resistance (TCR) is a critical variable in any thick film resistor. Because thick film resistor compositions are comprised of a functional (conductive) phase and a permanent binder phase, the properties of the conductive and binder phases and their interactions with each other and with the substrate affect both resistivity and TCR.
In the formulation of thick film resistor compositions for particular applications, it is often found that the TCR for the anticipated temperature range in use is too high and it therefore becomes necessary to increase or reduce the TCR in order that the resistivity not change too much over the operating range of temperature. It is well known in the thick film resistor art that additions of small amounts of various inorganic compounds will accomplish this. For example, in ruthenium-based resistors, it is known to employ for this purpose CdO, Nb.sub.2 O.sub.5, TiO.sub.2, MnO.sub.2, Mn.sub.2 O.sub.3, V.sub.2 O.sub.5 NiO, Sb.sub.2 O.sub.3 and Sb.sub.2 O.sub.5, all of which are negative TCR "drivers"; that is, they reduce TCR. On the other hand, CuO is known as a positive TCR driver in ruthenium-based resistors.
In the usual formulation of resistors, it is found that negative TCR drivers lower TCR, but simultaneously raise resistivity. Conversely, positive TCR drivers raise TCR, but lower resistivity. More recently, U.S. Pat. No. 4,362,656 to Hormadaly discloses the use of various manganese vanadates as TCR drivers for ruthenium-based resistors which have the advantage that they reduce TCR with only modest increases in resistance. It is interesting to note, however, that heretofore there is no known instance of a positive HTCR driver which also raises resistance. Moreover, it would be highly advantageous to have a TCR driver system with which both low positive and negative TCR's might be obtained in a resistor composition without raising resistance too much.